1. Field of the Invention
The invention relates to an implant to sheath and/or reinforce bone and/or to anchor bone screws.
2. Description of the Prior Art
Various healing forms can be identified in bone fractures and bone gaps. In accordance with the histological picture, a distinction is made between spontaneous bone healing and direct bone formation. The major differences are that in spontaneous bone healing cartilage and fibrous connective tissue are formed; the latter is converted to woven bone and lamellar bone tissue by means of re-differentiation In direct bone formation woven bones are formed directly in the case of large gaps and lamellar bones in the case of small gaps as a result of the high mechanical stability. The advantages of direct bone healing are due to the shortened healing time, i.e. bone bridging is achieved at an early stage. This guarantees very early mobilization and stability during exercise.
When fractures are treated conservatively, it is primarily spontaneous bone healing which occurs. This is why complications often result. These may include delayed or non-existent bone healing or non-union.
Generally speaking, direct bone healing cannot be achieved without operative repair and stable fixation by means of screws and plates. The interfragmentary compression required to guarantee mechanical stability is achieved by pre-loading the plate with a plate tension device or by means of the dynamic compression plate and an interfragmentary screw, in particular a screw with a tension or gliding hole, or also by means of "fixateurs externes".
These operation techniques were developed primarily by the Association for the Study of Internal Fixation. The histological picture of this type of bone healing is direct bone healing with no or very little callus formation. Bone grows through the remodelling units of the Haversian canals at the level of the fracture line. Bone screws with pre-cut threads have a firm support in the compact cortical bone. Cancellous bone screws with a widely projecting thread and a nearly rectangular pressure rest are used in the metaphyseal bone sections filled with cancellous bone. These screws are meant to interlock in the trabeculae.
Tensile tests and torque measurements have shown, however, that the cancellous bone in the metaphyses and the areas near the joints does not present the screws with any resistance whatsoever. As a result, the screws cannot build up any tension in the elastic cancellous bone. The tensile stress produced by the screws is eliminated within a few seconds. The screws cannot serve to fix the fragments mechanically, since they do not exert any interfragmentary compression. This is of particular practical importance when poorly positioned fragment ends are to be fixed one on top of the other by means of interfragmentary tension emanating from the screws. This is often necessary in certain types of fractures of the femoral neck, for instance. A necrosis of the entire epiphyseal section, i.e. necrosis of the head of the femoral neck, frequently results from insufficient fixation in this region. This often makes prosthetic replacements necessary, even in young patients. Only in very young individuals do cancellous bones screws have any support in the cancellous bone of the metaphyses, thus making it possible to demonstrate tensile stress for a number of minutes.
In older and elderly individuals, the trabeculae are very often scarce, so that even normal screws cannot be supported in the compact substance. In such cases, many attempts have been made to reinforce the osteosynthesis with plates and screws by means of an association with bone cement. The medullary cavity was filled with bone cement and the bone screws were fixed in the hardened cement. The drawback involved in this method, in particular for younger patients, is the fact that the bone cement cannot be removed. It remains in the medullary cavity section as a foreign body for the rest of the patient's life.
Similar problems are encountered when treating epiphyseal closures. The epiphyseal cartilage can also be locked contralaterally in unilateral epiphyseal fractures which often lead to unequal growth. Screw fixation sometimes suffices for this purpose, but frequently only partial closure is achieved as a result of unequal tension from the screws. This in turn leads to scoliosis. Similar problems occur when arthrodeses, i.e. artificial ankyloses, are performed. These are performed in cases in which joint movability is no longer desired since it disturbs joint stability. This type of arthrodesis has only been achieved in rare cases with screws, since in the course of time the screws loosened. Similar problems are involved in preventing the risk of microfractures in supporting epiphyseal structures or the risk of necroses in the head of the femoral neck. This is generally followed by the complete destruction of the hip joint. For example, depressed bone parts are supported or lifted, or torn bony tendon or ligament insertions are re-inserted post-traumatically.
Thus, the problem to be solved by the invention is to prepare surgical material which guarantees the secure fixation of bone screws and plates, in particular in bone sections near the joints. With this material, endangered or depressed bone parts are to be supported.
The problem is solved in particular by the features of the patent claims.